This invention relates to an improved process for the nitration of halogenated aromatic compounds whereby the ortho to para isomer distribution is increased.
Nitrochlorobenzenes are valuable chemical intermediates in the synthesis of dyes, photographic developers, anti-oxidants and gum inhibitors. Millions of pounds are produced each year to satisfy these needs.
The present methods used in the nitration of halogenated aromatics, for example the nitration of monochlorobenzene, yield a mixture of the para- and ortho-isomers, and place great emphasis on obtaining the para-isomer since it has, in the past, been in greater demand in the market.
The prior art discloses a number of processes for the nitration of aromatic compounds, but these processes are generally directed toward achieving a para-directive effect. As an example, U.S. Pat. No. 3,077,502 discloses that in the nitration of a halobenzene, a sulfonic acid produces a para-directive effect. Further, U.S. Pat. No. 3,140,319 discloses a method of increasing the amount of para-isomer in a nitrohalobenzene nitration product by nitrating the halobenzene with nitric acid in the presence of sulfuric acid, and purports that phosphoric acid can be used in lieu of sulfuric acid to obtain the same effect.
As can be seen from a review of the prior art, the nitration of chlorobenzene historically was conducted under such conditions to maximize the para-isomer formation. Although the para-isomer of a nitrated haloaromatic is necessary for many industrial purposes, the ortho-isomer is necessary for many other industrial purposes, and market demands for the ortho- and para-isomers have been changing. Flexibility in production is now necessary to meet increasing demands in the marketplace for the ortho-isomer, without a corresponding increase in the production of the para-isomer. The present invention solves this problem of controlling the para- to ortho-isomer distribution without increasing the meta-isomer formation or the formation of dinitrated products.